470R:
If that Ruger No.1 .500 Nitro belonging to your mate (PP..?)is the Vinny-engraved Pedersen-built rifle commissioned by Norm C_, everyone up here agreed that it was bloody brutal to shoot with full loads! I never fired it, but the maker, and Dave L_, both experienced big-bore users, had NO nice words to say about that rifle with full loads! Its an easy cartridge to load down, though, as you say.

Trex:
Plenty of good advice here, and the Lott makes most sense, but I would never chip a fellow shooter for preferring a classic cartridge, even at greater expense. Nevertheless, hiding a can of powder, dies, and a box of .450 Woodleighs at the mate's place, and slipping over for a 'social visit' every couple of weeks, would seem preferrable to rebarreling to .470 and then paying 12 bucks a round!